This invention relates generally to improved techniques for encapsulating objects such as electronic devices and components and, more particularly, to an improved method for the plastic encapsulation of semiconductor devices.
Plastic encapsulation of semiconductor devices by transfer molding is a well known and much used technique. In a typical situation, a large number of components or devices are placed in an open multi-cavity mold with one or more devices in each cavity. For manufacturing convenience, the devices are often coupled together on ladder-like lead frames or tapes. When the mold is closed, the two mold portions sealed against the lead frames are taped to prevent the leads from being covered with plastic. The many cavities in the mold are connected by a tree like array of channels (runners) to a central reservoir (pot) from which the plastic is fed. Typically, preheated plastic pellets are placed in the central reservoir (pot) and compressed by a ram or piston. The combination of heat and pressure causes the plastic to flow through the runner-tree into the individual mold cavities where it subsequently hardens.
Typically the plastic pellets are preheated in a radio frequency preheater until they become soft. Next, several of the preheated pellets are manually compressed causing them to stick together, and the group is manually lifted off the preheater and inserted into the reservoir or pot.
Unfortunately, to this point there has been no standardization of pellets and pot sizes. Due to this nonstandardization and due to the manual loading procedure of preheated pellets, air gets trapped in the compound during the initial stage of molding as a result of the RAM compressing the pellets. This trapped air eventually flows into the mold cavities with the plastic, is further compressed by the final mode compression and may show up as voids (defects which open to the surface), bubbles (normally invisible from the surface), and/or blisters (bubbles so close to the surface that they cause surface irregularities) in the molded semiconductor packages.
Thus, a need exists for an improved molding procedure and method for handling the plastic pellets so as to reduce the number of voids, bubbles and/or blisters which appear in the molded semiconductor package.